Liquid crystals (LC) are widely used for electronic displays. In these display systems, an LC layer is typically situated between a pair of polarizer layers. Incident light polarized by the first polarizer passes through a liquid crystal cell and is affected by the molecular orientation in the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes. Accordingly, LC technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light weight, low power consumption and long operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about the normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
Liquid crystal displays (LCDs) are quickly replacing CRT (Cathode Ray Tubes) as monitors for desktop computers and other office or house hold equipments. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCD's application as a replacement of the traditional CRT will be limited.
To correct this problem, several methods have been proposed. One of them is to place phase retardation films between the liquid crystal cell and polarizers of LCDs. The films compensate the phase retardation suffered by a light ray in a liquid crystal cell thus enlarging the viewing angle range with a good image quality. The second method is to utilize a particular construction of liquid crystal cell. For example, in a multi-domain mode, compensation is achieved by appropriately dividing the liquid crystal alignment in each pixel into a multiple of sub-pixels.
In these methods, the electric field is applied perpendicular to the surface of the liquid crystal cell to control the direction of the optic axis (the direction in which light does not see birefringence) of liquid crystal molecules. This means that the brightness variation in conventional LCDs is caused mainly by the change in the liquid crystal optic axis direction in the plane perpendicular to the liquid crystal cell surface. This is the main source of strong viewing angle dependence as rays propagating in various directions suffer different phase retardations.
In U.S. Pat. No. 5,600,464 Ohe et al. discloses a mode in which the electric field is applied in the plane (henceforth called in-plane field) of the liquid crystal cell. This mode is usually referred as an IPS mode as an abbreviation of “In Plane Switching” mode. In the IPS mode liquid crystal display, the liquid crystal optic axis changes its direction while remaining in the plane of the liquid crystal cell. This in return results in good viewing angle characteristic. The viewing angle characteristic (VAC) describes a change in a contrast ratio from different viewing direction. Here the viewing direction is defined as a set of polar viewing α and azimuthal viewing angles β as shown in the FIG. 1 with respect to a liquid crystal display 101. The polar viewing angle α is measured from display normal direction 103 and the azimuthal viewing angle β spans between an appropriate reference direction 105 in the plane of the display surface 107 and projection 108 of vector 109 onto the display surface 107. Various display image properties, such as contrast ratio, color and brightness are functions of angles α and β.
There are several possible types of operation in the IPS mode liquid crystal display. Lien et al. (SID Digest 1996, page 175-178) suggested different types in which liquid crystals initially take either homogeneous alignment or 90 degree twisted arrangement. They are explained according to the description referring to FIGS. 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B and 4C.
FIG. 2A is a top view of the homogeneous type IPS mode liquid crystal cell in the OFF state, a state without an in-plane field. The liquid crystal optic axis 201 orients homogeneously in the direction parallel to the transmission axis of the polarizer 203. The transmission axes 203, 204 of a pair of polarizers are crossed, where “crossed” means transmission axes form angle that is in the range 90±10°. A pair of electrodes 205A, 205B are connected to the voltage source 207 with a switch 209. The switch 209 shown in all figures is a simplified schematic. The switch shown in the figure represents the general switching elements such as thin film transistor (TFT) commonly used to turn on or off a single pixel in active matrix displays. FIGS. 2B and 2C show the ON state, a state with an in-plane field. Depending on the sign of dielectric anisotropy of the liquid crystals, they orient parallel (positive anisotropy, see FIG. 2B) or perpendicular (negative anisotropy, see FIG. 2C) to the in-plane field direction indicated by an arrow 202.
FIGS. 3A, 3B and 3C are side views corresponding to FIGS. 2A, 2B and 2C, respectively. The homogenous type IPS mode liquid crystal display 301 has two glass plates 309A and 309B. The first polarizer 302 is attached to the glass plate 309A and the second polarizer 303 is attached to the plate 309B. FIG. 3A is the OFF state where the liquid crystal optic axis 304 is parallel to the transmission axis of the first polarizer 302. In FIG. 3B, the liquid crystal optic axis 304 orients parallel to the direction of the in-plane field 305 generated between the electrodes 307A and 307B while perpendicular to the direction of the in-plane field 305 in FIG. 3C. The sign of dielectric anisotropy is positive in FIG. 3B and negative in FIG. 3C. Since the liquid crystal optic axis 304 is parallel to the transmission axis of the first polarizer 302 in the OFF state (FIGS. 2A and 3A), the light does not see birefringence and is blocked by the second polarizer 303. Thus the OFF state gives a dark state. While in the ON state (FIGS. 2B, 2C, 3B, and 3C), the optic axis 304 deviates with some angle from its original direction and the incoming light experiences phase retardation. As a result, the out coming light is no longer linearly polarized. Thus some portion of light goes through the second polarizer 303. This corresponds to the bright state. In actual applications, however, the liquid crystal optic axis 304 has non-zero tilt φ with respect to a the liquid crystal cell plane 310 (plane parallel to the surfaces of glass pates such as 309A, 309B) as shown in FIG. 3D for the OFF state. This is a result of the alignment procedure such as by a mechanical rubbing and usually less than 10 degrees. We define the azimuthal direction 311 of liquid crystal optic axis 304 by its projection on to the cell plane 310. So, for the OFF (dark) state, it is the azimuthal direction 311 of liquid crystal optic axis 304 that orients uniformly parallel to the transmission axis of the polarizer 302.
FIG. 4A shows a side view of a twist type IPS mode liquid crystal display in the OFF state. The twist type IPS mode liquid crystal display 411 is formed from the bottom and the top glass plates 401A, 401B and a pair of polarizers 402 and 403. The first polarizer 402 is attached to the glass plate 401A and the second polarizer 403 is put on the glass plate 401B. In this case, the transmission axes of the pair of polarizers 402 and 403 are parallel, where “parallel” means the transmission axes of the pair of polarizers form angle in the range of 0±10°. The liquid crystal optic axis 405 exhibits 90° azimuthal twist. In FIG. 4A, the twist is right handed meaning that the liquid crystal optic axis 405 rotates counter clockwise as one follows the direction of increasing thickness indicated by an arrow 407. If it rotates clockwise in the thickness direction, it is called left handed twist. Upon the application of an in-plane field between two electrodes 413A, 413B, unwinding of the twist occurs. FIGS. 4B and 4C show states of liquid crystal schematically in the ON state with the in-plane field for positive and negative dielectric anisotropy, respectively. In the OFF state (FIG. 4A), the incoming light polarized in the direction of the transmission axis of the first polarizer 402 is rotated 90° and absorbed by the second polarizer 403. This gives a dark state. Whereas the On state, corresponding to FIGS. 4B and 4C, is a bright state as the light is no longer linearly polarized upon entering into the second polarizer 403 due to the unwinding of the twist configuration of the liquid crystal optic axis 405. Usually, the optic axis 405 has non-zero tilt φ with respect to the liquid crystal cell plane 441 (here it is parallel to the surfaces of the glass plates 401A, 401B). The azimuthal direction 406 of liquid crystal optic axis 405 is defined by taking the projection of optic axis 405 on the liquid crystal cell plane 441.
Several attempts have been made to improve the VAC of the IPS mode liquid crystal displays using phase retardation films. FIGS. 5A, 5B, 5C and 5D show various types of phase retardation film represented by index ellipsoids. In FIG. 5A, the direction of optic axis 503 lies in the plane 501. The corresponding index of refraction is extraordinary index ne. The other refractive index is ordinary index no. If ne>no, it is called positive A-film, or negative A-film, otherwise. FIG. 5A shows a positive case. When the optic axis 505 is perpendicular to the plane 501 such as shown in FIG. 5B, the film is usually called as a C-film. It is positive if ne>no, negative, otherwise. The example in FIG. 5B is positive. The A-film and C-film are uniaxial films as there are two different indices of refraction, ordinary no and extraordinary ne. In uniaxial media, one can thus use their optic axes to describe the films' crystallographic orientation. There are also biaxial cases such that all of three principal indices of refraction nx0, ny0 and nz0 are different as shown in FIG. 5C. The slow axis lies in the direction of largest index of refraction. In the example shown in FIG. 5C, the largest index is nz0, thus the slow axis 509 is perpendicular to the film plane 501. In general, the slow axis 511 can point anywhere with respect to the film plane 501 as shown in FIG. 5D. In biaxial media, the optic axis is not necessarily parallel to the slow axis. However, we will use slow axis to describe the orientation of biaxial media in the following.
U.S. Pat. No. 6,184,957 discloses the use of film with negative birefringence (ne<no) for the IPS mode liquid crystal display. When combined with the IPS mode liquid crystal cell, the method prevents inversion of gradation and coloration in particular viewing angles. FIG. 6A shows an application of this method to the twist type IPS mode liquid crystal display 631. A twist type IPS mode liquid crystal cell 601 is sandwiched by a polarizer 603 and a film 605. Another polarizer 607 is placed on top of the film 605. The film 605 in this case consists of a negatively birefringent material and its optic axis exhibits azimuthal twist of 90° in the plane parallel to the film surface (twist-structured film). The sense of twist in the film 605 in this example is chosen to be opposite to that of the liquid crystal cell. The transmission axes 609, 611 of the pair of polarizers are crossed in this case. The VAC of the display in FIG. 6A is shown in FIG. 6B. Circles 621, 623 and 625 indicate the contrast ratio 400, 800 and 1200, respectively. The line 627 traces the change in the contrast ratio for azimuthal viewing angle 0°≦β<360° at a polar viewing angle α=30°. The compensation method shows some improvement compared to un-compensated case but contrast ratio drops when β is around 45°, 135°, 225°, or 315°.
In another attempt to optically compensate the IPS mode liquid crystal display, Saitoh et al. (SID digest 1998 page 706-709) proposed improvement on the homogeneous type IPS mode liquid crystal display. This was accomplished by placing a phase retardation film between the homogeneous type IPS mode liquid crystal cell and the polarizer. The homogenous type IPS mode liquid crystal display 717 is shown in FIG. 7A. A single biaxial compensation film 701 was used in combination with the homogeneous type IPS mode liquid crystal cell 703 and a pair of polarizers 705 and 707. The direction of slow axis 709 of the biaxial compensation film 701 is parallel to the direction of transmission axis 711 of the polarizer 705 and the azimuthal direction of liquid crystal optic axis 713 of the homogeneous type IPS mode liquid crystal cell 703 in the OFF state. The transmission axis 711 of the other polarizer 705 is perpendicular to the transmission axis 715 of the other polarizer 707. The configuration reduces the light leakage in OFF state and thereby increases the contrast ratio. However, this method has a severe limitation in cases with non-zero tilt angle φ as it is the case in FIG. 3D. As mentioned above, the liquid crystal optic axis 304 has non-zero tilt angle φ with respect to the liquid crystal cell plane 310 as a result of the alignment procedure in actual applications. This leads to a light leakage in the OFF state and results in degraded contrast ratio. FIGS. 7B and 7C are polar plots of a contrast ratio between the OFF and the ON state for φ=2° and φ=4°, respectively. The changes in a contrast ratio 721, 731 (here, contrast ratios, 300, 600 and 900 correspond to concentric circles, 723, 725 and 727, respectively) are plotted against the range of the azimuthal viewing angle 0°≦β<360° for the polar viewing angle α=30°. They demonstrate the degradation in contrast ratio in a wide range of azimuthal viewing angle β caused by the increase in the tilt angle φ of the liquid crystal optic axis. For example, the contrast ratio reduces to less than 300 in φ=4° tilt case (FIG. 7C) from 550 in φ=2° tilt (FIG. 7B) at β=150° and 330°.
The above-mentioned IPS modes and compensation methods have improved the VAC of liquid crystal displays to some extent. After careful examination, however, inventors realized that the prior art IPS mode liquid crystal displays have not attained satisfying viewing quality. This is mainly because of the degradation in contrast ratio in high viewing angle. It is partly attributed to the light leakage in the OFF state caused by the non-negligible tilt angle φ at which the liquid crystal optic axis is aligned with respect to the liquid crystal cell plane. Also, a light leakage from crossed polarizers lowers the viewing quality.
It is a problem to be solved to improve the VAC of an IPS nematic liquid crystal display.